This invention relates to magnetrons and more particularly to anode structures for use in magnetrons.
Magnetrons are a well known class of microwave tube and typically comprise a central cathode surrounded by a cylindrical anode structure which defines a plurality of resonant cavities. For example, the anode structure may comprise a cylindrical anode ring within which are located a plurality of radially disposed anode vanes.
Magnetrons may be used to generate microwave radiation over a range of frequencies depending on the geometry and dimensions of the anode structure. However, magnetrons are generally considered unsuitable for use in generating low frequency radiation, for example, frequencies of 400 MHZ or lower. Although these lower frequencies may be achieved by scaling up a conventional magnetron design this results in a device which occupies a large volume and is also unacceptably heavy and mechanically weak. Not only must increased amounts of materials be used to make up a larger device in any case, but also the various components must also be more massive to resist mechanical stresses imposed by a larger design and to withstand the vacuum required.
The present invention seeks to provide a magnetron, and an anode structure for use in such a magnetron which is able to operate at relatively low frequencies but is also a relatively compact and low weight structure.
According to a first aspect of the invention, there is provided an anode structure for a magnetron including a cylindrical member; and anode vanes disposed within the cylindrical member which define resonant cavities, each anode vane having a radially extensive portion, with an inner end and outer end, which adjoins the cylindrical member at its outer end and which is of substantially the same thickness at the outer end as that of the other anode vanes; and wherein each of a plurality of the anode vanes has a substantially radially extensive first portion and a second portion at its inner end which is extensive in a substantially circumferential direction.
According to a second aspect of the invention there is provided an anode structure for a magnetron, including a cylindrical member; and a plurality of anode vanes disposed within the cylindrical member which define resonant cavities, each anode vane disposed within the cylindrical member having a substantially radially extensive first portion with an inner end and an outer end and a second portion at the inner end which is extensive in a substantially circumferential direction.
According to a third aspect of the invention there in provided an anode structure for a magnetron including a cylindrical member; and anode vanes disposed within the cylindrical member which define resonant cavities, wherein each anode vane of a first set of the anode vanes has a substantially radially extensive first portion, with an inner end and an outer end, and has a second portion at its inner end which is extensive in a substantially circumferential direction; and wherein each anode vane of a second set of said anode vanes has only a substantially radially extensive portion which is of a substantially uniform thickness; and anode vanes of the first set being arranged alternately within the cylindrical member with anode vanes of the second set.
According to a fourth aspect of the invention there is provided an anode structure for a magnetron including a cylindrical member, anode vanes disposed within the cylindrical member which define resonant cavities; and wherein each anode vane of a plurality of the anode vanes has a substantially radially extensive first portion, with an inner end and an outer end, and a second portion at its inner end which is extensive in a substantially circumferential direction and at one of its ends adjoins said first portion.
In a conventional magnetron, the anode vanes include only radially extensive portions. In an anode structure in accordance with any of the aspects of the invention, the second portion of the anode vanes effectively increases the current path length around the anode cavities, thus increasing inductance in the anode structure. As the operating frequency of the magnetron is proportional to the reciprocal of the square root of inductance multiplied by capacitance, any increase in the inductance achieved by using the invention has the effect of lowering the operating frequency of the magnetron. Thus, for a given overall diameter of the anode structure and the same number of anode cavities, a significantly lower operating frequency may be achieved by employing the invention in comparison with a convectional structure.
In one advantageous embodiment of the first and second aspects of the invention for example, the first portions of at least some of the plurality join the respective second portions at the mid-point along the length of the second portion. This gives a xe2x80x9cT-shapexe2x80x9d anode vane. A T-shape configuration of anode vanes is advantageous because of the symmetry it offers. However, some aspects of the invention may be implemented using anode vanes which are an xe2x80x9cL-shapexe2x80x9d for example. Each of these may be arranged around the circumference of the cylindrical anode member in the same orientation or in another arrangement, the orientation of alternate L-shape anode vanes might be reversed, for example.
In a particularly advantageous embodiment of the first aspect of the invention for example, the plurality includes all anode vanes of the anode structure. This arrangement preserves a high degree of symmetry and a relatively large increase in inductance. However, for some applications it may be desirable, for example, to alternate a first set of anode vanes having a circumferential portion with a second set of anode vanes which are of a conventional configuration, being merely radially extensive in accordance with the third aspect of the invention.
Advantageously, more than two anode straps are included at one end of the anode structure. It is further preferred that more than two anode straps are included at each end of the anode structure. Preferably, four anode straps are included at at least one end of the anode structure. In other configurations, three, or more than four, anode straps may be included at at least one end of the anode structure.
The use of multiple anode straps in place of the usually provided two anode straps permits a large capacitance to be achieved in the anode circuit. Capacitance exists between facing surfaces of the anode straps and by employing more than two anode straps, this capacitance may thus be increased without needing to alter the dimensions or spacing of the straps from what would normally be considered suitable. Capacitance is also added between the surfaces of the anode straps and facing surfaces of the anode vane. Thus, capacitance may be increased by increasing the facing surface areas in the anode circuit without giving rise to the difficulties related to tolerance or problems with electrical breakdown which would arise if it were attempted to move the straps closer together to achieve an increase in capacitance The increase in capacitance compared to a conventional structure of the same overall dimensions gives a reduction in the magnetron operating frequency.
In one advantageous arrangement in accordance with the invention, at least one of the anode straps has a gap in its circumference located at the second portion of one of the anode vanes of the plurality. One or more gaps may be included in an anode strap without affecting its usefulness in achieving mode separation as the greater length in the circumferential direction of the vane as compared to a conventional purely radial vane permits the strap to be securely mounted in good electrical contact with the vane and also accommodate a gap. However, this leads to some reduction in capacitance and may not always be acceptable.
According to a first feature of the invention, a magnetron includes an anode structure in accordance with any aspect of the invention and a cathode is located coaxially within the anode structure.
A magnetron in accordance with the invention may be less than one thirtieth of the weight of a scaled up conventional magnetron for operation at the same frequency. As a further comparison, the reduction in diameter achievable making use of the invention leads to an anode structure of 264 mm diameter in comparison with a diameter of 1.2 m for a conventional magnetron for operation at the same frequency of 100 MHZ.
A further reduction in frequency may be achieved by providing a high magnetic field between the anode structure and the cathode. Preferably, the magnetic field strength is in the range of 500 Gauss to 2000 Gauss where the operating frequency of the magnetron is in the range of approximately 100 MHZ to 400 MHZ. As the operating frequency increases, an increase in magnetic field is required. As a comparison, for operation at 100 to 400 MHZ, in a conventional design, it would be expected to use a magnetic field of approximately 100 Gauss to 400 Gauss.
According to a second feature of the invention, a magnetron comprises means for producing a magnetic field between the anode structure and the cathode having a field strength in the range 500 Gauss to 2000 Gauss where the operating frequency of the magnetron is in the range of 100 MHZ to 400 MHZ.
In a particularly advantageous embodiment in accordance with the invention, the cylindrical member of the anode structure provides a return path for the magnetic field. In one arrangement, the cylindrical member includes steel with copper coating on its inner surface. This gives a compact structure in which it is not necessary to separately provide a magnetic return path.